The invention pertains to an apparatus for the rapid evacuation of a vacuum chamber by means of a first vacuum pump, preferably a Roots vacuum pump, and an intake line with a first shut-off valve connecting the intake port of this first pump to the vacuum chamber. A second vacuum pump is installed downline of the first pump by means of a connecting line. A bypass line connects the working chamber of the first vacuum pump to the connecting line and brings about a preintake cooling function. A blow-out valve is installed in this connecting line.
For the rapid evacuation of large volumes, pump stands with preintake-cooled Roots vacuum pumps are frequently used. In chambers which are to be evacuated to the pressure range below 200 mbars, multi-stage pump stations have been found useful. It is known that a Roots vacuum pump can be used as the largest pump connected directly to the vacuum chamber and that the following pump stage can be any desired combination of preintake-cooled Roots vacuum pumps and/or other pumps. For the evacuation process, the largest preintake-cooled Roots vacuum pump is connected to the vacuum chamber. Thus a powerful suction capacity is achieved starting right at atmospheric pressure. As a result of this method, the downline (smaller) pumps can no longer transport the quantity of gas conveyed by the first pump once the pressure falls below atmospheric pressure. To prevent the buildup of an undesirable positive pressure in this case, a blow-out valve leading to the outside is usually installed between the first and the second pump stage. Depending on the staging of the selected pumps, a transition pressure is obtained, from which pressure on the blow-out valve is closed, because the fore-pumps are now able to convey the mass flow conveyed by the first stage in the negative pressure range. The fore-pump stand has an effect on the total suction capacity only below the transition pressure. At higher pressures, the fore-pump stand therefore remains unused.